1. Field of the Invention
The present invention relates to an infrared detector, and in particular to a passive infrared (PIR) detector. Such infrared detectors find application in intruder detection systems and fire alarms.
2. Discussion of Prior Art
By way of background to the present invention, conventional passive infrared (PIR) detectors typically utilise pyroelectric detector elements comprising a lead-based ceramic material, for example lead zirconate titanate (PZT). The lead-based ceramic material used within the detector elements is relatively easy to process and the associated manufacturing technology is mature. Many tens of millions of PIR detectors are made every year at a typical cost of around 1 US dollar per detector element.
Despite the foregoing, conventional lead-based PIR detectors do suffer several potential disadvantages.
Conventional PIR sensors typically comprise a sub-assembly having several discrete components including a PIR detector element. The PIR detector element is cut from pressed ceramic material and undergoes several processing steps whereby the thickness is reduced and detector is polished. The detector element is subsequently mounted in a package and interfaced to a read-out transistor. An infrared (IR) transparent window or lens is usually fitted to the PIR sensor. Processing and assembly of the various components into the PIR sub-assembly is protracted and requires the separate manufacture of the detector in bulk, fabrication of the detector itself, and assembly of the detector with an amplifier chip. Hence, the discrete manufacturing steps associated with processing, packaging and interfacing the PIR detector element conspire to increase the overall cost of the PIR sub-assembly.
With regard to technical performance, operating parameters for conventional PIR detectors are largely governed by the properties of the PZT material from which the detector element is made, and the degree to which it is thermally isolated from its substrate. Variability within the composition of the material PZT can affect detector performance, resulting in poorly defined and variable operating parameters, e.g. time constant etc.
The performance of the PIR detector is also a function of the design of the detector element, which is fixed at the time of manufacture. Accordingly, key operating parameters (e.g. field of view) for conventional PIR detectors cannot be altered after manufacture, nor tailored easily to meet end-users' specific requirements.
Another factor which may affect the continued use of conventional lead-based PIR detectors is the present desire to eliminate heavy metals from electrical and electronic equipment. Indeed, European Community Directive 2002/95/EC on the restriction of the use of certain hazardous substances (RoHS) in electrical and electronic equipment will be enforced throughout the European Community (EU) from 1 Jul. 2006. Manufacturers of conventional lead-based PIR detectors will thus have to find an alternative form of detector, which does not contain lead, to supply existing PIR detector markets.
Several alternatives to conventional lead-based PIR detectors exist, although such alternatives are aimed at different applications to conventional PIR detectors. For example, Lithium Tantalate may be used as an alternative material within a PIR detector. However, Lithium Tantalate is traditionally used for applications requiring high performance and accordingly the high cost of the material leads to its use in high-quality longer range detection systems, which also have a higher cost.
Bolometer detectors provide a further alternative class of infrared detectors. Bolometer detectors are traditionally aimed at medium to high performance civil and military infrared imaging applications and are usually configured in a two-dimensional focal plane array (FPA). Such a bolometer array is traditionally operated in a vacuum within an hermetically sealed enclosure. The technology to provide long-term sealing of the hermetic enclosure is expensive. Further, the array may need to be temperature stabilised, and the absolute temperature controlled carefully, to ensure adequate performance. Complex signal processing electronics are usually required to decode the multiplexed output from this type of detector and to correct for non-uniformity in the outputs from the detector elements within the array.
Mindful of the foregoing, bolometer detectors are not considered as a credible alternative to conventional PZT PIR detectors on grounds of disparate technical performance and cost.
It is an object of the invention to provide an alternative infrared detector. It is a further object of the invention to provide an infrared detector which mitigates at least some of the disadvantages of conventional PIR detectors described above.